Relentless Evolution: The vital relevance of the visible

A medium ground-finch (Geospiza fortis), one of the flagship examples of “relentless evolution.”

One of Stephen Jay Gould’s sharpest conceptual coinages was a barb leveled, from his paleontological perspective, at the body of research focused on bouts of adaptive evolution occurring over “ecological” timespans on the order of a few generations. Reviewing such cases as replicated evolutionary experiments with E. coli, and the evolution of limb length in Anolis lizards relocated to tiny, lizard-free islands, Gould acknowledged that such cases are important for determining the scope and frequency of change driven by natural selection. But he objected strongly to the idea that such rapid changes were the “atoms” from which million-year evolutionary trends are built:

… if a case of evolution proceeds with sufficient speed to be discerned by our instruments in just a few years—that is, if the change becomes substantial enough to stand out as a genuine and directional effect above the random fluctuations of nature’s stable variation and our inevitable errors of measurement—then we have witnessed something far too substantial to serve as an atom of steady incrementation in a paleontological trend.

Gould’s “paradox of the visibly irrelevent” holds that, if we are to understand the river of evolutionary history, we must look below the spume and spray of year-to-year adaptative change to find the deeper currents that can, over time, carve canyons. In his new book Relentless Evolution (University of Chicago Press, $35.00 in paperback), John N. Thompson makes the opposing argument with gusto: To Thompson, studying the roiling eddies that Gould dimissed as transient and superficial is the only way to understand the deeper currents, and the river’s course ahead of us.

The central thesis of Relentless Evolution is that evolutionary change driven by natural selection is continual, ubiquitous, and significant in the year-to-year dynamics of any population of living things. The book builds its vision a piece at a time—from sources of natural selection to its effects on gene frequencies within individual species to coevolutionary responses in other interacting species; from the origins of local adaptation across variable landscapes to the emergence of new species and, ultimately, the assembly of communities.

Thompson stuffs every chapter with case studies, citing research dating to the foundational work of the Modern Synthesis as well as the latest in evolutionary ecology. The breadth and depth of scholarship covered in Relentless Evolution will make it especially valuable for any beginning graduate student surveying the field for a niche in which to develop a dissertation. However, the book should be of interest to anyone—from research faculty to informed non-scientists—looking for a survey of the current state of evolutionary ecology and the historical origins of key concepts.

Here are just some of the species whose patterns of adaptation are discussed in Relentless Evolution: Water fleas. Influenza viruses. Atlantic cod. Wild and cultivated flax. Dogs. Sticklebacks. Swainson’s thrushes. Bobtail squid. Darwin’s finches. The microbiota of Homo sapiens. Goldenrod and goldenrod gallflies. Pseudomonas aeruginosa. Pine trees and crossbills. Joshua trees. Caenorhabditis elegans. Whole assemblages of tropical forest trees. And, of course, Thompson’s beloved Greya moths, the nature of whose relationship with the plants they pollinate, and in which they lay their eggs, is contingent on the presence of other pollinators. The text dances from species to species without slowing, clearly explaining necessary background information before fitting each study system into its place within Thompson’s vision of slowly building complexity.

Given Thompson’s commitment to his one-element-at-a-time organizational scheme, this thoroughness can often make Relentless Evolution feel more like a textbook than a sustained, focused argument—and, paradoxically, this can mean that some important nuances are lost in the shuffle. One example is the treatment of genetic architecture: how many genes contribute to traits of ecological importance, the distribution of their effects, and the nature of interactions between them. These factors directly determine whether a population can respond to selection, and the rate at which it will—yet Thompson moves quickly past the real concerns that current methods for identifying quantitative trait loci are biased towards loci of large effect to discuss the many well-documented examples of adaptation via variation at such “Mendelian” loci.

Relentless Evolution can also, in places, feel like a forest of evolutionary possibilities with no path leading to predictions for how given population or species might actually evolve. When Thompson writes, on page 153, “What we know, though, can sound complicated, because it is complicated and we know only some parts of the answers,” he is referring specifically to parasites’ effects on their hosts’ demography—but he could be speaking for different topics in virtually every chapter in the book.

However, the core message of Relentless Evolution—which emerges, like an evolutionary trend, from patient and continuous accumulation of detail—is that ecologists and evolutionary biologists must understand precisely these complications. Thompson draws explicit connections from the multi-species metapopulation dynamics of his “geographic mosaic of coevolution” to patterns of speciation and extinction in adaptive radiation. Contra Gould, he holds that short-term adaptive changes in individual, local populations are, ultimately, the fuel that sustains the tempo and mode of diversification over geological epochs. More immediately, Thompson notes that the spatially variable adaptive dynamics that fascinate him are also the forces that will determine what portion of Earth’s biological diversity survives the unprecedented changes we humans have made, and are making, to our world.

In the argument with Gould, then, Thompson’s winning riposte is to say yes, the relentless churnings of adaptation may be mere eddies in the great river of evolution—but those eddies are where we live.